The ‘old’ theming method with CSS files is still available, however, we
switched to using scss on many files which is likely to break your theme.
Re-creating your theme following the usual documentation should suffice.
To lower the effort you need to spend on theming your Nextcloud instance we
highly recommend to use the theming app instead of the themes feature. See
Theming for details.

In the Nginx configuration the Same-Origin header was removed. This is now
handled in PHP and keeping it will result in a wrong header. Please remove
those lines in your Nginx config:
add_headerX-Frame-Options"SAMEORIGIN";
See Nginx Configuration

The following error message is shown during the update: “Repair warning: Could
not install core app bundle: Could not download app <app>”.

This basically means that Nextcloud could not fetch the app from the appstore
automatically. This could have multiple reasons: either you disabled the
appstore with the config.php flag or your server could not reach the app store.
The instance will work fine, but the features that are usually provided by this
app are not available.

With Nextcloud 12, CSS files have been merged into one server.css so in order
to keep your theme working you should consolidate your existing css styles into
a server.css file. As for the example theme the styles.css file has been
renamed to server.css.